Optical deflector

ABSTRACT

An optical deflector comprises a control means for controlling at least either a light source or a deflection means adapted to gauge the distance between the position of a deflected beam of light moving on a light receiving element in one direction and the position of another deflected beam of light moving on the light receiving element in the opposite direction and control the distance so as to make it agree with a predetermined value. Thus, the optical deflector can very accurately control the operation of the deflection means in such a way that it is not affected by changes of environmental temperature of the deflection means and the detection circuit, because a detection means for detecting the time when a beam of light passes by a predetermined angle of deflection of the deflection means is not used.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an optical deflector having a deflection meansfor deflecting light.

2. Related Background Art

FIG. 1 of the accompanying drawings illustrates a galvano-mirror as anexample of optical deflector that is driven by electromagnetic force. Amirror is arranged on a movable section, which is supported by a mainbody by way of a pair of torsion bars so that it may be rotated relativeto a central axis. In FIG. 1, reference symbol 50 denotes a siliconsubstrate and reference symbols 51 and 52 respectively denote an upperglass substrate and a lower glass substrate. There are also shown amovable plate 53, a pair of torsion bars 54, a planar coil 55, a totalreflection mirror 56, a pair of electrode terminals 57 and permanentmagnets 60 through 63. The illustrated optical deflector is of theelectromagnetic type that is driven by causing a drive current to flowthrough the planar coil 55 and utilizing the Lorentz force that isgenerated by the drive current and the permanent magnets (see, interalia, U.S. Pat. No. 5,606,447).

Japanese Patent Application Laid-Open No. 2001-305471 describes anelectromagnetic actuator. This patent document has much in common withU.S. Pat. No. 5,606,447 in that a movable part is driven byelectromagnetic force. The electromagnetic actuator disclosed inJapanese Patent Application Laid-Open No. 2001-305471 also has a totalreflection mirror arranged on a movable part.

Japanese Patent Application Laid-Open No. 2001-305471 describes a systemas follows in terms of problems, objects and means. The inventiondisclosed in the above patent document paid attention to the fact thatthe resonance period of an electromagnetic actuator normally drifts withtemperature and time and hence, if an electric current having apredetermined, constant resonance frequency is continuously supplied tothe planar coil, there arises a problem that it is not possible tocontrol the angle of deflection and keep it to a constant value withtemperature change and time lapse. Thus, the first object of thatinvention is to provide an electromagnetic actuator that can be drivento reciprocate with its resonance period without providing a separatedetection means as well as a drive control device and a drive controlmethod to be used for such an electromagnetic actuator. The secondobject of that invention is to provide an electromagnetic actuator whoseangle of deflection can be controlled without providing a separatedetection means as well as a drive control device and a drive controlmethod to be used for such an electromagnetic actuator. The third objectof the invention is to provide a resonance frequency signal generatingdevice and a resonance frequency signal generating method to be used foran electromagnetic actuator that can output a resonance frequency signalcorresponding to the resonance period of the electromagnetic actuator.

The invention of the above cited patent document utilizes a coil asmeans for exciting the movable section of the electromagnetic actuatorand also as detection means. The induced voltage or current in the coilis utilized for detection.

While Japanese Patent Application Laid-Open No. 2001-305471 describesthat the resonance period of an electromagnetic actuator drifts withtemperature and time, it proposes to detect the time when the actuatorpasses by a predetermined angle of revolution (deflection) from the coilthat is a detection means as time-related information.

U.S. Pat. No. 5,606,447 does not pay attention to the problem that theresonance period of an electromagnetic actuator drifts with temperature.

With the method of detecting the time when the actuator passes by apredetermined angle of deflection gives rise to a signal delay in thedetection circuit due to changes of environmental temperature.Additionally, timing errors can occur when gauging the change with timeof the angle of deflection and detecting the time on the basis of thegauged change because the detection timing can be shifted by signalnoises and offsets.

Thus, the problem of signal delays and timing errors arises whenaccurately controlling an actuator by such a method of detecting thetime when the actuator passes by a predetermined angle of deflection.

SUMMARY OF THE INVENTION

The inventor of the present invention came up with an idea differentfrom that of the inventor of the invention disclosed in Japanese PatentApplication Laid-Open No. 2001-305471. More specifically, it is theobject of the present invention to make it possible to very accuratelycontrol the operation of a deflection means, or an actuator, withoutusing a detection means for detecting the time when a beam of lightpasses by a predetermined angle of deflection, or actuator, in such away that it is not affected by changes of environmental temperature ofthe deflection means and the detection circuit.

Thus, according to the invention, there is provided an optical deflectorhaving a deflection means for deflecting modulated light from a lightsource so as to make deflected beams of light scan, said opticaldeflector comprising a control means for gauging a distance between aposition of a deflected beam of light moving on a light receivingelement in one direction and a position of another deflected beam oflight moving on the light receiving element in the opposite directionand controlling at least either the light source or the deflection meansso as to make the distance agree with a predetermined value.

In another aspect of the invention, there is provided a method ofcontrolling an optical deflector adapted to deflect light from a lightsource so as to make deflected beams of light scan, the methodcomprising: a position detecting step of detecting a position of adeflected beam of light moving on a light receiving element in onedirection and a position of another deflected beam of light moving onthe light receiving element in the opposite direction; a step of sensinga phase difference between a phase as detected in the detecting step anda predefined phase; and a step of controlling either a drive frequencybeing applied to the optical deflector or a modulation timing forreciprocative drawing according to an outcome of the sensing step.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an optical deflector;

FIG. 2 is a schematic illustration of scanning of beams of light thatare deflected (reflected) by the deflection means of the firstembodiment of optical deflector so as to reciprocate on a lightreceiving element, showing the deflected beams of light scanning only inone direction along with the trajectory thereof;

FIG. 3 is a schematic cross sectional view of the first embodiment ofoptical deflector taken along a plane containing a beam of lightdeflected by the deflection means;

FIGS. 4A, 4B and 4C are graphs illustrating exemplary drive waveformsthat can be applied to the deflection means 202 of the first embodimentof optical deflector;

FIG. 5 is a schematic block diagram of the first embodiment of opticaldeflector, illustrating the control flows thereof;

FIGS. 6A, 6B and 6C are schematic illustrations of the secondembodiment;

FIG. 7 is a schematic illustration of a plurality of light receivingelements that the light receiving element of the third embodimentincludes;

FIG. 8 is a schematic cross sectional view of the fourth embodiment ofoptical deflector taken along a plane containing a beam of lightdeflected by the deflection means;

FIGS. 9A and 9B are graphs illustrating the frequency characteristics ofthe resonance-type deflector of the fifth embodiment of opticaldeflector;

FIGS. 10A and 10B are graphs illustrating drive signal 309 of thedeflection means 202 of the resonance-type deflector of the fifthembodiment of optical deflector and the change with time of the angle ofdeflection at the time when the drive signal 309 is applied;

FIGS. 11A and 11B are schematic illustrations of a method of generatingmodulated spots by the sixth embodiment of optical deflector;

FIGS. 12A and 12B are schematic illustration of another method ofgenerating modulated spots by the sixth embodiment of optical deflector;

FIG. 13 is a schematic illustration of the seventh embodiment of opticaldeflector;

FIG. 14 is a schematic illustration of the light receiving element 101arranged within a scanning area 214 and the display region of theseventh embodiment of optical deflector;

FIG. 15 is a schematic illustration of the eighth embodiment of opticaldeflector;

FIG. 16 is a schematic illustration of the configuration of the deviceof Example 1;

FIG. 17 is a flow chart of the operation of Example 1;

FIG. 18 is a flow chart of the operation of Example 2;

FIGS. 19A and 19B are schematic illustrations of the configuration ofthe apparatus of Example 3; and

FIG. 20 is a flow chart of the operation of Example 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment

The inventor of the present invention came to have an idea of utilizingdeflected beams of light that are emitted from a light source anddeflected to reciprocate for scanning by a deflection means that alsoreciprocates (swings), for controlling at least either the light sourceor the deflection means.

More specifically, according to the invention, deflected beams of lightmoving forward and moving backward are detected by a light receivingelement and either the deflection means or the light source iscontrolled by way of a control means in such a way that the distance(displacement) between the position of the forwardly moving deflectedbeam of light and that of the backwardly moving deflected beam of light,each at a given clock time, shows a predetermined value.

FIG. 2 is a schematic illustration of scanning of beams of light thatare deflected (reflected) by the deflection means of the firstembodiment of optical deflector so as to reciprocate on a lightreceiving element, showing the deflected beams of light along with thetrajectory thereof.

In FIG. 2, reference symbol 101 denotes the light receiving element andreference symbols 102 and 103 denote the deflected beams of light, whilereference symbol 104 denotes the trajectory of the deflected beams oflight.

The deflected beam of light 102 moves along the trajectory 104 indirection A. The other deflected beam of light 103 moves along thetrajectory 104 in direction B. Both the deflected beam of light 102 andthe deflected beam of light 103 pass by the light receiving element 101.

The light receiving element 101 is arranged at a predetermined positionwhere it can detect (receive) both the deflected beam of light movingforward and the deflected beam of light moving backward. Provided thatthe deflected beam of light 102 moving forward and the deflected beam oflight 103 moving backward passing by the receiving element 101 takerespective positions that are different from each other at a given clocktime for each, the distance between the positions of the deflected beamsof light 102, 103 is the distance of displacement.

With this embodiment, at least either the deflection means or the lightsource is controlled by a control means, which will be described ingreater detail hereinafter, in such a way that the displacement is foundwithin an appropriate range of displacement (predetermined distance).

The embodiment will be described in greater detail below.

Firstly, the positional displacement between the position of the beam oflight moving forward and that of the beam of light moving backward thatare deflected by a deflection means will be discussed.

FIG. 3 is a schematic cross sectional view of the first embodiment ofoptical deflector taken along a plane containing beams of lightdeflected by the deflection means.

In FIG. 3, there are shown a light source 201, a deflection means 202, abeam of light 203 emitted from a light source, beams of light 204, 205that are deflected by the deflection means 202 with the largest angle ofdeflection, the central axis 206 of optical deflection of the deflectionmeans 202 and the scanning trajectory 207 on plane P that is separatedfrom the deflection means 202 by distance L (plane perpendicular to thecentral axis 206 of optical deflection).

The beam of light 203 emitted from the light source 201 is made tostrike the deflection means 202. A light source that is adapted tomodulation such as semiconductor laser is used for the light source 201.

The deflection means 202 is provided with a reflection plane so that itdeflects a beam of light within the largest angle of deflection asindicated by the beams of light 204, 205 as it is driven to move. Thelargest angle of deflection is denoted by θ.

In the following description, it is assumed that the reflected beam oflight is found on the central axis 206 of optical deflection when thedeflection means 202 is not driven to move.

The deflection means 202 is driven to rotate around the rotary axis andreciprocate. A periodical drive waveform is applied to the deflectionmeans.

FIGS. 4A through 4C are graphs illustrating exemplary drive waveformsthat can be applied to the deflection means 202 of the first embodiment.The horizontal axis represents the duration of application and thevertical axis represents the amplitude of the applied signal. FIG. 4Ashows a triangular waveform and FIG. 4B shows a saw-edged waveform,while FIG. 4C shows a sinusoidal waveform. The deflection means 202changes the angle of deflection corresponding to the waveform of theapplied signal.

As the deflection means 202 is driven to deflect a beam of light byapplying a periodical signal as shown in any of FIGS. 4A through 4C, thescanning position on the plane P that is separated from the deflectionmeans 202 by distance L (the movement of a deflected beam of light on aplane is referred to as scanning hereinafter) reciprocates on thescanning trajectory 207. The scanning position h (the distance from thecentral axis 206 of optical deflection on the plane P that is separatedfrom the deflection means 202 by distance L) can be expressed by theequation below;h=L× tan (θ(t))  (1),where θ(t) represents the angle of deflection by which the beam of lightis deflected from the central axis 206 of optical deflection at a givenclock time.

It may be assumed here that the light source 201 is operated formodulation and the position of the forwardly moving deflected beam oflight 102 and that of the backwardly moving deflected beam of light 103at a given clock time for each are not displaced from each other.

However, the scanning characteristic of the deflection means 202 canchange because of the components change due to temperature changes andthe drive means delays. Therefore, if the same waveform is applied tothe deflection means at the same timing and the light source 201 isoperated for modulation at the same clock time, the timing of deflection(scanning) of the forwardly moving and backwardly moving beams of lightmay be displaced and hence the position of the forwardly movingdeflected beam of light 102 and that of the backwardly moving deflectedbeam of light 103 at the given clock time for each may be displaced fromeach other on the light receiving element 101.

With this embodiment, it is hence possible to sense a change in thedisplacement of the scanning positions of the forwardly moving andbackwardly moving beams of light due to changes of various factorsrelating to the embodiment including environmental factors (changes inthe scanning timings) by detecting a relative displacement between theposition of the forwardly moving deflected beam of light and that of thebackwardly moving deflected beam of light at a given clock time foreach.

Note that, while the positions of the deflected beams of light 102, 103change when the largest angle of deflection θ changes due to variousenvironmental factors, no relative positional displacement of the twodeflected beams of light occurs so long as the scanning timing does notchange. Therefore, it is possible to detect the relative displacement ofthe scanning positions of the forwardly moving and backwardly movingdeflected beams of light without being influenced by the change in thelargest angle of deflection θ.

Now, the method of detecting the relative displacement of the scanningpositions of the forwardly moving and backwardly moving deflected beamsof light by means of a light receiving element will be described below.

First, a technique of generating modulated spots to be used fordetecting the position of a deflected beam of light by means of a lightreceiving element will be discussed.

The light receiving element 101 is arranged on the scanning trajectory207 located on the plane P that is separated from the deflection means202 by distance L. Any position may be selected for the light receivingelement 101 so long as it is found on the scanning trajectory 207. Forthe purpose of simplicity of description, assume here that the lightreceiving element 101 is arranged substantially at the center of thescanning area.

In order to detect the relative displacement of the scanning positionsof the forwardly moving and backwardly moving deflected beams of lightat a given clock time for each, a technique of deflecting a modulatedbeam of light that is obtained by turning on and off the light source toform regions where luminance of light (quantity of light) showsdistribution (to be referred to as modulated spot hereinafter) by meansof the light receiving element 101 and gauging the positional gapbetween the modulated spots will be employed. More specifically, asshown in FIG. 2, the light source is turned on and off once at a givenclock time within a period of time when a beam of light is caused toforwardly scan on the light receiving element 101 and also once within aperiod of time when a beam of light is caused to backwardly scan on thelight receiving element 101 to produce spots (high luminance spotsgenerated by a scanning beam of light) 102, 103 respectively on theforward moving path and the backward moving path.

As a result, it is possible to detect the positions of the deflectedbeams of light at a given clock time for each by observing thedistribution of the total quantity of the electric charge induced bylight on the light receiving element 101.

Thus, the relative positional displacement of the modulated spots can begauged by means of the light receiving element 101 that is adapted tooutput a signal that allows the gap between the spots 102, 103 to begauged on the light receiving element 101.

Second, the light receiving element 101 that is adapted to output asignal that allows the gap to be gauged on the element itself will bediscussed.

The light receiving element 101 of this embodiment is required to detectthe position of each modulated and deflected beam of light as positionalinformation and also the gap separating the positions of two deflectedbeams of light.

A line sensor (image sensor) comprising a plurality of light receivingregions 105 can be used for the light receiving element 101 of thisembodiment. Such an arrangement requires each light receiving region tocomprise a light receiving element section that operates asphotoelectric transducer, an accumulating section for accumulating theelectric charges that are obtained as a result of photoelectricconversion and a transfer section for transferring the accumulatedelectric charge.

Then, the position of a deflected beam of light can be accuratelyidentified because each of the plurality of light receiving regions candetect the quantity of the deflected beam of light.

Then, it is not necessary to transfer the accumulated electric charge ata high rate corresponding to the scanning speed. Rather, the electriccharge can be transferred at a lower rate after the formation ofmodulated spots on the forward and backward moving paths on the lightreceiving element 101. Therefore, the relative positional displacementof the modulated spots can advantageously be detected if the scanningspeed v is raised on the light receiving element 101 (e.g., if theperiod of application of a waveform is reduced).

When such a light receiving element is used, the distribution ofquantity of light in each of the modulated spots on the light receivingelement 101 is accumulated in the form of electric charge and output aspositional information on each of the plurality of light receivingregions 105. Thus, this embodiment is free from the problem of a reducedaccuracy of detection that arises to the method of directly detectingthe scanning (deflection) timing because of changes in the delay of thedetection circuit. Hence, this embodiment can highly accurately detectthe relative positional displacement of the modulated spots.

This embodiment employs a method of detecting the forwardly movingmodulated and deflected beam of light and the backwardly movingmodulated and deflected beam of light by means of a light receivingelement and gauging the change in the timing of forward scanning andbackward scanning as positional information by referring to therespective scanning positions at a given clock time for each. Therefore,there does not arise any problem of lowered detection accuracy that canbe caused by a delay in the detection circuit of the prior art and henceit is possible to highly accurately detect any change in the scanningcondition.

Next, the method of controlling the detected relative positionaldisplacement between of the forwardly moving and backwardly movingdeflected beams of light so as to keep it to a constant value will bediscussed.

FIG. 5 is a schematic block diagram of this embodiment of opticaldeflector, illustrating the control means thereof.

In FIG. 5, there are shown a deflected beam of light 208, a modulationsignal generation means 301 for the light source 201, a modulationsignal 305 for the light source 201, a detection signal 306 from thelight receiving element, a signal conversion means 302, a scanningposition displacement signal 307, a control signal generation means 303,a control signal 308 for the deflection means 202, a drive means 304 forthe deflection means 202, a drive signal 309 for the deflection means202 and a modulation control signal 310 for the light source 201.

The light source 201 is driven to turn on and off repeatedly (formodulation) by a modulation signal 305 from the modulation signalgeneration means 301 that operates to turn on and off the light source201 with a predetermined period. The modulated beam of light 203 isdeflected by the deflection means 202 and detected by the lightreceiving element 101. The modulation signal 305 is generated so as todetect modulated spots 102, 103 respectively in the forward scanningdirection and in the backward scanning direction. The information on themodulated spots in the forward and backward scanning directions detectedby the light receiving element 101 is transmitted to the signalconversion means 302 as output signal 306.

The signal conversion means 302 computationally determines the relativepositional displacement between the scanning positions of the forwardlymoving modulated beam of light and the backwardly moving modulated beamof light on the basis of the detection signal 306 from the lightreceiving element and outputs a scanning position displacement signal307 that represents the relative displacement of the scanning positionsof the modulated and deflected beams of light.

The control signal generation means 303 modifies either the controlsignal 308 for the deflection means 202 or the modulation control signal310 for the light source 201 in such a way that the relative positionaldisplacement of the scanning positions of the modulated and deflectedbeams of light becomes equal to a predetermined value (e.g., 0) on thebasis of the scanning position displacement signal 307.

The control signal 308 of the deflection means 202 is adapted to changethe rate at which the movable plate swings in order to change the timingof deflection of the mirror (movable plate) that belongs to thedeflection means 202.

The drive means 304 selects a timing or period for the drive signalaccording to the control signal 308 and applies the drive signal 309 tothe deflection means 202, or the deflector.

The modulation control signal 310 for the light source 201 is adapted toadjust the timing or period of the modulation signal 305 so that themodulation of light may be in harmony with the deflection timing of themirror (movable plate) that belongs to the deflection means.

It may be so arranged to change either the control signal 308 for thedeflection means 202 or the modulation control signal 310 for the lightsource 201 or both of them.

Thus, this embodiment of optical deflector can control the relativepositional displacement between the forwardly scanning beam of light andthe backwardly scanning beam of light so as to keep it a constant levelby detecting the modulated and deflected beams of light by means of alight receiving element.

Thus, by using the above described detection/control method, there isprovided a method of driving an optical deflector having a deflectionmeans for deflecting a modulated beam of light from a light source, themethod comprising:

a gauging step of gauging the distance between the position of adeflected beam of light moving on a light receiving element in adirection and the position of another deflected beam of light moving inthe opposite direction; and

a control step of controlling either the light source or the deflectionmeans so as to make the distance take a predetermined value by a controlmeans.

This embodiment is not limited to the above described configuration andmay be modified in various different ways as will be described below.

For instance, it may be so arranged as to transfer the accumulatedelectric charge after generating a forwardly moving modulated spot onthe light receiving element 101, generate a backwardly moving modulatedspot after the transfer and subsequently resume the transfer of electriccharge. With this arrangement, the scanning operation of the deflectionmeans needs to be stabilized within a short period of time (which is atleast twice as long as the time required for transferring the electriccharge). Then, the forwardly moving modulated spot and the backwardlymoving modulated spot can be detected separately and hence it ispossible to use a simplified algorithm to detect the positionaldisplacement.

While zero is used above as exemplary value for defining the relativepositional displacement between the forwardly moving modulated spot andthe backwardly moving modulated spot for this embodiment, anotherappropriately selected value may alternatively be used. Then, theforwardly moving modulated spot and the backwardly moving modulated spotare separated from each other so that they may be detected with ease.

While the light receiving element 101 is arranged on the scanningtrajectory in the above-described embodiment, it may alternatively bearranged at some other position and a mirror may be additionallyprovided so as to reflect the deflected beams of light moving on thescanning trajectory. Then, the light receiving element 101 detects thedeflected beams of light that have been reflected by the mirror. Theprovision of a mirror alleviates the positional restrictions imposed onthe light receiving element 101 so that an optical deflector of thisembodiment can be downsized.

While only a single light source is used in this embodiment, the presentinvention is applicable to an optical deflector having a plurality oflight sources. Only one of the plurality of light sources may be usedfor the purpose of the invention.

Any light source that is adapted to modulate the beam of light emittedfrom it can be used for this embodiment. Examples of such light sourcesinclude semiconductor lasers, LEDs, solid lasers and gas lasers having amodulation means such as AOM.

While this embodiment is described above in terms of one-dimensionaloptical scanning where a forwardly moving deflected beam of light andthe backwardly moving deflected beam of light pass along a sametrajectory, the present invention is also applicable to so-calledtwo-dimensional optical scanning where the backwardly moving deflectedbeam of light proceeds along a trajectory that is perpendicularlyseparate from the trajectory along which the forwardly moving deflectedbeam of light proceeds.

This embodiment is described above in terms of deflected beams of lightadapted to one-dimensional scanning. However, this embodiment can beused for an exposure device adapted to emit light onto the cylindricalphotosensitive body of an electrophotography-type image formingapparatus so as to produce an electrostatic latent image bytwo-dimensionally scanning the surface of the photosensitive body bymeans of deflected beams of light when the deflected beams of light aremade to scan the revolving cylindrical photosensitive body along thelongitudinal direction thereof.

This embodiment can also be used for a projection-type image displayapparatus such as a projector when the deflected beams of light are madeto scan two-dimensionally.

In such an image forming or image display apparatus, the light is turnedon and off corresponding to the pixels of the image being produced. Thesize of each pixel is not particularly limited. In other words, it isdefined according to the image to be produced by the apparatus. In eachpixel, not only the actual scanning spot diameter but also its shapechange as a function of the scanning distance because the scanning spotmoves in one direction while the light source is on, though the changedepends on the scanning speed. If a light source whose quantity of lightdiffers between the center of light emitting point and the peripheralarea (e.g., to show a Gaussian distribution) is used, the practicalpixel size may be regarded to be equal to that of the region where alarge quantity of light is found (e.g., a half of the largest quantityof light or 1/e²) is found regardless if the pixel moves in onedirection while the light source is on. In the case of a projector, forexample, that produces an image that human eyes can directly watch, thesize of the pixel that moves while the light source is on may be definedas such, taking the human vision into consideration.

With this embodiment of optical deflector, the relative positionaldisplacement of the forwardly moving modulated pattern and thebackwardly moving modulated pattern can be eliminated on the projectionsurface (that is scanned by beams of light) without being influenced bythe change in the scanning timing of the deflection means 202 when therelative positional displacement of the forwardly moving deflected beamof light and the backwardly moving deflected beam of light that aremoving on the light receiving element is so controlled as to show aconstant value.

Thus, when this embodiment is used for an exposure device adapted toemit light onto the photosensitive body of an electrophotography-typeimage forming apparatus or a display apparatus of the above-describedtype that is adapted to display an image on a two-dimensional displayscreen, it can display a desired image by using both a forwardly movingscanning beam of light and a backwardly moving scanning beam of light toimprove the exposure rate or the display speed, whichever appropriate.

Second Embodiment

This embodiment differs from the first embodiment in terms of the methodfor identifying the position (central position) of each of the modulatedand deflected beams of light (modulated spots) on the light receivingelement 101 comprising a plurality of light receiving regions 105.Otherwise, this embodiment is identical with the first embodiment.

FIGS. 6A, 6B and 6C are schematic illustrations of the secondembodiment.

FIG. 6A shows the positional relationship of the position on the lightreceiving element 101 where a modulated spot is formed and the pluralityof light receiving regions 105. Note that the scanning direction ishorizontal on FIG. 6A. For the purpose of simplicity, assume here thatthere is a single modulated spot on the light receiving element 101. Forthe purpose of the invention, the process that is described below may berepeated for the number of times that is equal to the number of spotsthat is involved in the optical deflector.

FIG. 6B is a graph illustrating the distribution of quantity of lightirradiated onto the light receiving element 101 when the positionalrelationship of FIG. 6A is applicable. In FIG. 6B, the horizontal axisrepresents the position on the light receiving element and the verticalaxis represents the quantity of light. The modulated spot shows adistribution pattern that is symmetrical relative to position C wherethe quantity of light is largest (which may be a Gaussian distributionpattern in which the center is light and the peripheral area is dark).

FIG. 6C shows the signals that are detected in the plurality of lightreceiving regions 105 when the positional relationship of FIG. 6A isapplicable. In FIG. 6C, the horizontal axis represents the position ofeach of the plurality of light receiving regions that corresponds to aselected position on the light receiving element and the vertical axisrepresents the magnitude of the detected signal. In the followingdescription, the width of each of the plurality of light receivingregions that runs in the scanning direction is assumed to be w and thequantity of light detected on the n-th light receiving region from theleft end in FIG. 6B is assumed to be Pn.

Two position identifying methods will be described below by referring toFIGS. 6A through 6C.

With the first position identifying method, a positional coordinatesystem is defined on the basis of the light receiving region thatobtained the largest quantity of light in the plurality of lightreceiving regions. For example, in the case of FIG. 6C, the 5th lightreceiving region takes the role of the basis of a positional coordinatesystem and the center of the modulated spot is detected so as to belocated at the position of [(5−0.5)×w].

The detection accuracy is determined by the relationship between thescanning velocity v on the light receiving element 101 and the width wof each of the plurality of light receiving regions 105 that runs in thescanning direction. If the scanning velocity v is constant, thedetection accuracy rises as the width w of each of the plurality oflight receiving regions that runs in the scanning direction is reduced.

With the first position identifying method, a simple process can be usedto identify a particular position on the light receiving element so thatboth the time necessary for the process and the load borne by thecomponents responsible for the process can be reduced. Additionally, thedetection accuracy can be improved by reducing the width w of each ofthe plurality of light receiving regions 105 that runs in the scanningdirection.

With the second position identifying method, a position on the lightreceiving element is identified on the basis of the distribution ofdetection signals obtained on the plurality of light receiving regions.The distribution of quantity of light of the modulated spot detected onthe light receiving element can be regarded to be symmetrical relativeto position C where the quantity of light is largest. Thus, if theposition C where the quantity of light is largest does not agree withthe center of one of the plurality of light receiving regions 105 or theboundary of one of the light receiving regions 105, the detectionsignals from the light receiving regions 105 are arranged asymmetrically(see the detection signals of FIG. 6C as an example). Using thisphenomenon, the positional coordinate of each of the light receivingregions [(n−1)×w] is multiplied by the quantity of received light Pn andthe products of the multiplications are added to obtain the total sum[Σ(n−1)×w×Pn]. Then, the total sum is divided by the total quantity ofreceived light [SPn] in the light receiving region where the spot exists([Σn−1)×w×Pn]/ΣPn) to identify the positional coordinate. Thus, it isnow possible to detect the center of the spot with a level of errorsmaller than the width w of each of the plurality of light receivingregions. With this method, it is preferable that the deflected beam oflight extends on more than one plurality of light receiving regions 105from the viewpoint of identifying the position of the deflected beam oflight. This method can be used when the position C where the quantity oflight is largest does not agree with the center of one of the pluralityof light receiving regions 105 or the boundary of one of the lightreceiving regions 105.

With the above described second method, it is possible to provide aresolution that is smaller than the width w of each of a plurality oflight receiving regions 105 so that the position of the modulated spotcan be identified highly accurately if a light receiving element 101 inwhich the width w of each of a plurality of light receiving regions 105is relatively large (smaller than the width of the modulated spot to bedetected) is used.

Third Embodiment

This embodiment differs from the first and second embodiments in thatthe light receiving element 101 has a plurality of light receivingregions 105 that are two-dimensionally arranged. Otherwise, thisembodiment is identical with the first and second embodiments.

FIG. 7 is a schematic illustration of a plurality of light receivingelements that the light receiving element of the third embodimentincludes.

As shown in FIG. 7, the light receiving element 101 has a plurality oflight receiving regions 105 that are arranged two-dimensionally. Inother words, the light receiving regions 105 have a square profile andare arranged in rows and columns.

When a plurality of light receiving regions 105 are arrangedtwo-dimensionally as in the case of this embodiment, the distribution ofquantity of light of a modulated spot can be detected two-dimensionallyso that the profile of the modulated spot can be grasped accurately anda large quantity of data can be used for selecting either of theposition identifying methods described above in terms of the secondembodiment.

Thus, the position of the modulated spot can be identified highlyaccurately by using this embodiment.

The light receiving element 101 of this embodiment can be realized byusing a general purpose CCD area sensor or CMOS area sensor that findsapplications in imaging operations so that it is not necessary to designa particular sensor for the light receiving element 101. Thus, thisembodiment can be realized at low cost.

While the light receiving element of this embodiment has a plurality oflight receiving regions having a square profile and arranged in rows andcolumns in the above description, it may alternatively have aconfiguration where a plurality of light receiving regions are arrangedto show a honeycomb, a configuration where the rows or the columns oflight receiving regions are alternately indented and offset in thedirection of the trajectory or in a direction perpendicular to thetrajectory, whichever appropriate, or a configuration where the lightreceiving regions have a circular, parallelogramic, triangular, rhombic,trapezoidal or polygonal profile.

Fourth Embodiment

In this embodiment, deflected beams of light entering the lightreceiving element is converged by a lens. Otherwise, this embodiment isidentical with the preceding first through third embodiments.

FIG. 8 is a schematic cross sectional view of the fourth embodiment ofoptical deflector taken along a plane containing a beam of lightdeflected by the deflection means. In FIG. 8, reference symbol 209denotes a lens.

The lens 209 is arranged at a position separated from the deflectionmeans by distance L. The light receiving element 101 is arranged at aposition separated from the lens 209 by a distance equal to the depth offocus f of the lens 209. The beam of light deflected by the deflectionmeans 202 shows a reduced width (diameter) when it is converged by thelens to form a modulated spot. Additionally, the beam of light isfurther deflected by the lens so that the center of the modulated spotis shifted from that of the modulated spot of the first embodiment.

The relationship between the scanning position h and the angle ofdeflection θt at time t is expressed by the formula below.h=f× tan (θt)  (2)

From the equation (2), it will be seen that the scanning velocity v onthe light receiving element 101 can be raised by selecting a large depthof focus f for the lens 209 regardless of the distance L. Then, the rateof change of the relative positional displacement of the forwardlymoving modulated spot and the backwardly moving modulated spot is raisedto improve the detection accuracy.

Additionally, the positions of the modulated spots do not depend on thedistance L. Therefore, the arrangement of the related components isfacilitated and the embodiment can be downsized when a small value isselected for the distance L and a lens 209 having a large depth of focusf is used.

The lens may be used as a component of this embodiment of opticaldeflector. Alternatively, the lens may be integrally arranged with thelight receiving element.

The light receiving element 101 can accurately identify the positions ofthe deflected beams of light when the deflected beams of light areconverged onto the light receiving element 101 by means of a lens 209 asin this embodiment. Additionally, the entire optical deflector can bedownsized.

Fifth Embodiment

This embodiment differs from the first embodiment in that it comprises adeflection means 202 that utilizes a resonance phenomenon. Otherwise,this embodiment is identical with the first embodiment.

When a resonance-type deflector is used for the deflection means 202, itis possible to provide a wide angle of deflection by making themechanical resonance frequency fc of the resonance-type deflector andthe drive frequency fd agree with each other if the drive energy is notraised. However, the mechanical resonance frequency fc of the deflectorcan change remarkably as a function of the change in the environmentalfactors of the deflector including ambient temperature and therefore thescanning (deflecting) timing of the deflector 202 changes.

Therefore, it is necessary to control the resonance frequency fc of theresonance-type optical deflector and the drive frequency fd so as tomake them agree with each other.

FIGS. 9A and 9B are graphs illustrating the frequency characteristics ofthe resonance-type deflector of the fifth embodiment of opticaldeflector.

In FIG. 9A, the horizontal axis represents the frequency fd of the drivesignal for swinging the resonance type deflector and the vertical axisrepresents the amplitude of the change in the angle of deflection(swinging angle) (largest angle of deflection θ) of the resonance-typedeflector. In FIG. 9A, the frequency that provides the largest value forthe largest angle of deflection θ is the resonance frequency fc (in anideal case where any delay in the drive circuit and/or some othercircuits does not need to be taken into consideration).

In FIG. 9B, the horizontal axis represents the frequency fd of the drivesignal for swinging the resonance type deflector (just like thehorizontal axis of FIG. 9A) and the vertical axis represents the phasedelay from the synchronizing signal of the drive frequency fd. Note thatthe origin (0 deg) on the horizontal axis of the phase delay can changedepending on how the synchronizing signal of the drive frequency fd isgenerated.

As seen from FIGS. 9A and 9B, the phase changes as the drive frequencyfd and the resonance frequency fc vary and hence the timing of scanningchanges in a resonance-type optical deflector.

The relationship between the two graphs is maintained when the resonancefrequency fc of the deflector is constant. The relationship between thegraphs of FIGS. 9A and 9B is maintained if the resonance frequency fcchanges (the profiles of the curves including the slopes and the widthsof the curves are held similar) and only the parameter of the drivefrequency fd represented by the horizontal axes of FIGS. 9A and 9Bchanges.

Thus, it is possible to make the drive frequency fd and the resonancefrequency fc agree with each other by driving the resonance-typedeflector with a drive frequency fd that constantly maintains (bychanging the drive frequency to maintain) a same scanning timing(phase).

The influence on the phase (scanning timing) increases as the value ofthe drive efficiency (Q value of resonance) rises, if the frequencydifference remains the same. Therefore, it is necessary to change thefrequency at a smaller pitch.

Thus, in this embodiment, it is possible to constantly maintain the samescanning timing by detecting the relative positional displacement of theforwardly moving modulated beam of light and the backwardly movingmodulated beam of light and controlling the drive frequency fd so as tomake it follow the resonance frequency fc of the resonance-type opticaldeflector.

FIGS. 10A and 10B are graphs illustrating drive signal 309 of thedeflection means 202 of the resonance-type optical deflector of thefifth embodiment and the change with time of the angle of deflection atthe time when the drive signal 309 is applied.

FIG. 10A shows the temporal change of the amplitude (e.g., of thevoltage) of the drive signal 309 of the deflection means 202. In FIG.10A, the horizontal axis represents time and the vertical axisrepresents the amplitude of the drive signal 309.

In FIG. 10B, the horizontal axis represents time (just like thehorizontal axis of FIG. 10A) and the vertical axis represents the angleof deflection θt of the deflection means 202 at time t.

If the drive signal shows a sinusoidal waveform and the drive frequencyfd and the resonance frequency fc agree with each other as shown in FIG.10A, a phase delay of 90 degrees occurs to the change of the angle ofdeflection θt as shown in FIG. 10B. A phase offset of 180 degrees canoccur depending on the selection of the sense of deflection of thedeflection means 202 (the definition of the positive side and thenegative side for a slope inclined in a direction relative to the rotaryaxis).

When a resonance-type optical deflector is used, the angle of deflection?t changes with time to show a sinusoidal waveform even if the waveformof the drive signal is not sinusoidal but triangular, rectangular orsaw-edged.

Therefore, since the scanning (deflection) time of the forwardly movingbeam of light and that of the backwardly moving beam of light are equalto each other, the time that can be used for the modulating operation ofthe light source 201 is shortened (to less than a half of the availabletime) and the efficiency of the use of light is reduced if only one ofthe scanning beams of light is utilized. The modulating operation of theresonance-type optical deflector needs to be conducted by using both theforwardly moving beam of light and the backwardly moving beam of lightin order to avoid the above identified problem.

With this embodiment, it is possible to maintain the relativedisplacement of the scanning position of the forwardly moving scanningbeam of light and that of the backwardly moving scanning beam of lightby using a resonance-type deflector for the deflection means 202. Thus,a resonance-type deflector that can produce a large angle of deflectionat a low power consumption rate can be used for an application whereboth a forwardly moving scanning beam of light and a backwardly movingscanning beam of light are utilized. As a result, it is possible toprovide an optical deflector whose power consumption rate is low andefficiency of utilization of light is high.

While the drive frequency fd of this embodiment is made to agree withthe resonance frequency fc in the above description, it is also possibleto make the drive frequency fd and the resonance frequency fc to alwaysshow a constant difference and hence a certain constant relationship. Ifsuch is the case, the ratio of the change in the scanning timingrelative to the frequency displacement becomes small to facilitate theoperation of controlling the drive frequency fd so as to make it followthe resonance frequency fc.

Sixth Embodiment

This embodiment differs from the first through fifth embodiments interms of the method for generating modulated spots and detecting theirrelative positional displacement and the method for detecting therelative positional displacement. Otherwise, it is identical with thepreceding first through fifth embodiments.

FIGS. 11A and 11B are schematic illustrations of a method of generatingmodulated spots by the sixth embodiment of optical deflector.

FIG. 11A shows a waveform that the drive signal 309 to be applied to thedeflection means 202 can take. In FIG. 11A, the horizontal axisrepresents time and the vertical axis represents the amplitude of theapplied signal. While the waveform of the drive signal is triangular, itis illustrated only as example.

FIG. 11B shows the modulation signal 305 to be used for modulating(turning ON and OFF) the light source 201. In FIG. 11B, the horizontalaxis represents time (just like the horizontal axis of FIG. 11A) and thevertical axis represents the pattern of the modulation signal 305. Notethat the modulation signal 305 is usually an OFF signal but it turns tobe an ON signal when the light source 201 is modulated to generate amodulated spot on the light receiving element 101.

The modulation signal 305 produces an ON signal once for each timeperiod, during which the deflection means 202 is driven to make both aforwardly moving beam of light and a backwardly moving beam of lightscan on a plane containing the light receiving element 101, andgenerates modulated spots 102, 103, which are separated from each other,on the light receiving element 101.

While the drive signal has a triangular waveform in the abovedescription, some other waveform may alternatively be used. FIGS. 12Aand 12B schematically illustrate a drive signal having a sinusoidalwaveform that can be used for generating modulated spots (both thevertical axes and the horizontal axes of FIGS. 12A and 12B are identicalwith their counterparts of FIGS. 11A and 11B).

While only a single forwardly or backwardly moving modulated spot isgenerated in the above description, a plurality of forwardly orbackwardly moving modulated spots may be alternatively be generated.When a plurality of modulated spots are detected by the light receivingelement 101, the relative positional displacements are gauged and theaverage of the positional displacements is determined and used toimprove the accuracy of detection.

When this embodiment of optical deflector is used for the exposuredevice of an image forming apparatus or an display device and therelative positional displacement of the forwardly moving modulated spotand the backwardly moving modulated spot is made to show a predeterminedvalue (which may be equal to 0), they should be controlled to operatefor exposure or display properly. Then, a high quality image is formedby using both forward scanning and backward scanning.

Seventh Embodiment

This embodiment differs from the first through sixth embodiments in thatthe optical deflector projects deflected beams of lighttwo-dimensionally on the light receiving surface. Otherwise, it isidentical with the preceding first through sixth embodiments.

FIG. 13 is a schematic illustration of the seventh embodiment of opticaldeflector.

In FIG. 13, there are shown a second deflection means 211, a scanningtrajectory 210 on the reflection plane of the second deflection means211 formed by the deflection means 202, beams of light 212 deflected bythe second deflection means 211, the scanning area 214 on a plane 213 inwhich deflected beams of light scan, and the trajectory 215 of scanningbeams of light on the plane 213.

Note that the arrangement for controlling the operation of the opticaldeflector as shown in FIG. 5 is not illustrated in FIG. 13.

Each of the deflection means 202 and the second deflection means 211 isadapted to deflect each beam of light both horizontally and vertically.Therefore, the deflected beams of light produced by the deflection meanscover a two-dimensional region.

The deflection means 202 and the second deflection means 211 haverespective deflection velocities that are different from each other.More specifically, when the two deflection means are compared with eachother in FIG. 13, the deflection means 202 deflects beams of light atrelatively high speed (high frequency), whereas the second deflectionmeans 211 deflects beams of light at relatively low speed (lowfrequency). The speed relationship may be inverted.

The deflection means that deflects beams of light at relatively highspeed can display highly fine images when a resonance-type deflector isused. This is because a resonance-type deflector is adapted to highspeed deflecting operations.

The beam of light 203 modulated by and emitted from the light source 201is deflected by the deflection means 202, the largest angle ofdeflection being defined by beam of light 204 and beam of light 205 (thelargest angle of deflection θ). The second deflection means 211 deflectsthe beams of light that scan the reflection plane along the trajectory210 to produce beams of light 212 that scan the plane 213 arranged at aselected position so as to define a scanning area as indicated by 214.Note that reference symbol 215 denotes the schematically illustratedtrajectory of scanning beams of light within the scanning area 214 onthe plane 213.

The light receiving element 101 is placed at an appropriate position inthe scanning area 214. More specifically, it is placed on a horizontalpart of the scanning trajectory.

FIG. 14 is a schematic illustration of the light receiving element 101arranged within the scanning area 214 and the display region of thelight receiving element 101 of this embodiment.

In FIG. 14, reference symbol 220 denotes the display region to be usedfor forming images.

The scanning area 214 includes a display region 220 and a region wherethe light receiving element 101 is arranged. As deflected beam of light212 starts scanning from scanning point S1, it moves back and forth inthe horizontal scanning direction X to gradually scan from upper part ofthe scanning area to lower part of the area along the vertical scanningdirection Y. When the deflected beam of light 212 gets to scanning pointS2, it is returned to scanning point S1 and repeats the same scanningcycle.

It is so arranged that the deflected beam of light 212 moves on thelight receiving element 101 that is placed on the scanning line 215.

With this embodiment of optical deflector that is applied to atwo-dimensional image forming apparatus comprising a resonance-typedeflector, the relative positional displacement of the modulated beamsof light is detected by the light receiving element 101 and controlledto show a predetermined desired value while the modulated beams of lightare forming an image. Thus, it is possible to display a high qualityimage by means of a resonance-type deflector that produces a forwardlymoving beam of light and a backwardly moving beam of light.

While the light receiving element 101 is arranged within the scanningarea 214 in the above description of the embodiment, scanning beams oflight, or deflected beams of light between the deflection means 202 andthe second deflection means 211, may be taken out from the scanning area214 for detection by means of a reflector mirror.

While the region where the light receiving element 101 is arranged andthe display region 220 are separated from each other in the abovedescription of the embodiment, a region for arranging the lightreceiving element 101 or a reflector mirror to be used by the lightreceiving element 101 to detect modulated spots may be provided withinthe display region 220 so long as the light receiving element 101 or thereflector mirror does not visually adversely affect the displayed image.

The deflected beam of light 212 may be so adapted that it becomes brightonly when it moves on the light receiving element 101. In other words,the deflected beam of light 212 is required to be bright at least on thepart of the trajectory located on the light receiving element 101.

Eighth Embodiment

This embodiment of optical deflector is characterized in that itcomprises a light receiving element 101. Otherwise, it is identical withthe first through seventh embodiments.

Like the seventh embodiment, this embodiment of optical deflector isapplied to an image forming apparatus and also comprises two deflectionmeans that are driven to produce respective deflected beams of light forhorizontal scanning and vertical scanning to display a two-dimensionalimage. Only the arrangement that differentiates this embodiment from theseventh embodiment will be discussed below.

FIG. 15 is a schematic illustration of this embodiment of opticaldeflector.

In FIG. 15, there are shown a frame body 213, a scanning region 214provided on the plane of the frame body, a display region 220 and thetrajectory 215 of scanning beams of light in the display region.

The light receiving element 101 is arranged in the scanning region 214on the frame body. Thus, the plane containing the display region can beseparated from the plane carrying the light receiving element.

Additionally, because the distance L from the deflection means 202 tothe light receiving element 101 and the distance from the central axis206 of optical deflection to the position of the light receiving element101 can be fixed, the timing of generating a modulation pattern can becomputed with ease on the basis of the arrangement of the lightreceiving element.

This embodiment of optical deflector can be applied to an image displayapparatus such as a projector of the front type with which the viewerwatches the image formed in the display region 220 from a positionlocated between the frame body and the image.

This embodiment allows to freely define the display region because thedisplay region onto which an image is projected can be placed on anyplane. In other words, the projector can be used with any plane becausethe plane on which an image is projected for displaying is not subjectedto any restrictions.

This embodiment of optical deflector can be applied to an image displayapparatus such as a rear projector when it is so arranged that theviewer watches the image formed in the display region 220 from the sideopposite to the display surface of the display region 220.

Additionally, this embodiment can also be applied to an image displayapparatus of the type adapted to display an image directly on theretinas of the viewer or a head-mount display type image formingapparatus.

While the frame body 213 is not an indispensable component of thisembodiment of optical deflector, the provision of a frame body 212 ispreferable because the light receiving element 101 can be aligned withease when it is rigidly secured to the frame body 213.

The use of a frame body 213 is also preferable for defining the displayregion 220. Therefore, a frame body 213 on which the light receivingelement 101 is arranged may be defined as an indispensable component ofthis embodiment.

While the term “modulation pattern” is used in the above description ofthe preferred embodiments, it will be paraphrased to “modulation patternfor detection” in the following description in order to discriminate itfrom a modulation pattern for drawing an image that is used for imageformation.

As for the scanning direction of the deflection means, scanning fromleft to right on the light receiving element 101 in FIG. 2 is defined tobe forwardly moving direction.

EXAMPLE 1

In Example 1, an optical deflector according to the invention is usedfor an exposure device adapted to emit light onto the photosensitivebody of an electrophotography-type image forming apparatus.

FIG. 16 is a schematic illustration of the configuration of the deviceof Example 1.

In FIG. 16, there are shown a light receiving element 101, a lightsource 201, a deflection means 202, a photosensitive drum 220, the axis221 of the photosensitive drum that includes a trajectory of scanning, abeam of light 203 emitted from the light source 201 and beams of light204, 205 that define the largest angle of deflection.

The beam of light 203 modulated by and emitted from the light source 201is deflected by the deflection means 202 to produce deflected beams oflight that scan forwardly and backwardly on the axis 221 respectively soas to form a desired modulation pattern on the photosensitive bodywithin a scanning period and expose the photosensitive body to light.The largest angle of deflection and the arrangement of the lightreceiving element 101 are so selected as to make it possible to detectmodulated spots on the light receiving element 101 (detect scanningbeams of light outside the photosensitive body).

The light source 201 is directly modulated by means of an infraredsemiconductor laser (λ=780 nm). The deflection means 202 is agalvano-mirror driven by a 10 kHz triangular wave. A 1/7 inch CMOS imagesensor (black and white sensor conforming to the CIF Specifications) isused for the light receiving element 101.

The modulation pattern is defined such that modulated spots are formedon the light receiving element 101 as shown in FIG. 2. As shown in FIG.2, the modulated spot that is scanning forwardly is located left to themodulated spot that is scanning backwardly and it is so arranged thatthe photosensitive body is exposed properly to the forwardly scanningbeam of light and the backwardly scanning beam of light when therelative positional displacement of the modulated spots gets topredetermined value Lg±α, where α is the tolerance for the accuracy offorward scanning and backward scanning of the photosensitive body.

The above operation is controlled by the arrangement illustrated in FIG.5.

FIG. 17 is a flow chart of the operation of this example.

As the control starts, firstly an initial drive frequency and an initialmodulation pattern for detection are defined (S101). Firstly, thedeflection means 202 is driven on the basis of the above information togenerate a modulation pattern for detection.

The modulation pattern for detection is generated on the light receivingelement 101 in such a way that it is turned on once for a forwardscanning period and once for a backward scanning period (S102). Afterthe generation of the modulated spots, the electric charge accumulatedin the light receiving element by the modulated spots is transferred insuch a way that the electric charge in one of the plurality of lightreceiving regions is transferred at a time (S103).

Then, the relative positional displacement of the modulated spots iscomputed on the basis of the transferred information (S104) by using themethod described above by referring to the second embodiment for thecomputation.

If the computed relative positional displacement is found within thepredetermined range of Lg±α, the steps from S102 are repeated. If, onthe other hand, it is not found within the predetermined range, it isdetermined if the relative positional displacement is greater or smallerthan Lg (S106).

If the relative positional displacement is greater than Lg, it isbecause the modulation timing is too quick and hence the gap separatingthe forwardly scanning modulated spot and the backwardly scanningmodulated spot is made too large. Therefore, the modulation timing ismade slower (S107).

If, on the other hand, the relative positional displacement is smallerthan Lg, it is because the modulation timing is too slow and hence thegap separating the forwardly scanning modulated spot and the backwardlyscanning modulated spot is made too small. Therefore, the modulationtiming is made quicker (S108).

When shifting the modulation timing, the positional relationship of themodulation pattern for detection and the modulation pattern on thephotosensitive body is maintained so that only the timing is madequicker or slower.

Thereafter, the processing operation returns to S102 to repeat the abovesteps.

In this way, the relative positional displacement of the forwardlyscanning modulated spot and the backwardly scanning modulated spot canbe held within a predetermined range. Thus, there arises no displacementbetween exposure to the forwardly scanning modulated spot and exposureto the backwardly scanning modulated spot on the photosensitive body. Inother words, the photosensitive body is exposed to light correctly andtherefore, it is possible to realize an electrophotography-type imageforming apparatus that produces fine images.

EXAMPLE 2

Example 2 differs from Example 1 in generation of modulation pattern fordetection and the method of transferring the accumulated electric chargefrom the light receiving element. Otherwise, this example is identicalwith the preceding example.

Unlike in Example 1, a modulation pattern for detection is not generatedfor forward scanning and for backward scanning successively and thevalue of Lg is equal to 0 in this example. In other words, if thephotosensitive body is exposed properly to the forwardly scanningmodulated spot and the backwardly scanning modulated spot (and hencethere is no relative positional displacement thereof), the position ofthe modulation pattern for detection for forward scanning agrees withthat of the modulation pattern for detection for backward scanning.

FIG. 18 is a flow chart of the operation of Example 2.

After initialization (S201), a single ON signal is generated asmodulation pattern for detection in a forward scanning period and amodulated spot is generated on the light receiving element 101 (S202).Thus, only a modulated spot is generated at position 102 in FIG. 2. Theelectric charge accumulated by the modulated spot in the forwardscanning period is transferred on a region by region basis (S203). Then,a single ON signal is generated as modulation pattern for detection in abackward scanning period and the accumulated electric charge istransferred (S204 and S205).

The positional relationship and the relative position displacement ofthe forwardly scanning modulated spot and the backwardly scanningmodulated spot are computed from the obtained positional information ofthose spots (S206). If the position displacement is found within thetolerance ±α, the steps from S202 are repeated. If, on the other hand,the positional displacement is not found within the tolerance, thepositional of the forwardly scanning modulated spot and that of thebackwardly scanning modulated spot are compared. Note that, the relativepositional displacement is defined to be positive when the forwardlyscanning modulated spot is found to the left of the backwardly scanningmodulated spot, whereas the relative positional displacement is definedto be negative when the forwardly scanning modulated spot is found tothe right of the backwardly scanning modulated spot (S208).

If the positional displacement is positive, it is because the modulationtiming is too quick and hence the forwardly scanning modulated spot isfound to the left of the backwardly scanning modulated spot. Therefore,the modulation timing is made slower (S209).

If, on the other hand, the relative positional displacement is negative,it is because the modulation timing is too slow and hence the forwardlyscanning modulated spot is found to the right of the backwardly scanningmodulated spot. Therefore, the modulation timing is made quicker (S210).

Thereafter, the processing operation returns to S201 to repeat the abovesteps.

In the case of this example, it is possible to separately detect theforwardly scanning modulated spot and the backwardly scanning modulatedspot on a light receiving element 101 having only a small region. Thus,the light receiving element 101 can be downsized to reduce the cost ofthe entire apparatus.

A time lag occurs between the generation of the forwardly movingmodulated spot for detection and that of the backwardly moving modulatedspot for detection (time period required for transferring theaccumulated electric charge). Therefore, the arrangement of this examplemay preferably be employed when a resonance type optical deflector whosescanning characteristics would not fluctuate in a short period of timeis used for the deflection means 202.

EXAMPLE 3

In Example 3, an optical deflector according to the invention is usedfor a laser scanning projection type display apparatus.

FIG. 19A is a schematic illustration of the configuration of theapparatus of this example.

In FIG. 19A, reference symbol 222 denotes a reflector mirror andreference symbol 209 denotes a converging lens. Otherwise, theconfiguration is identical with that of 8th Embodiment.

The light source 201 is directly modulated by means of a redsemiconductor laser (λ=635 nm). The deflection means 202 drives theresonance type optical deflector having a resonance frequency of 28 kHzby means of a rectangular wave. A CMOS image sensor having 100×100regions (each region is a 10 μm square) is used for the light receivingelement 101. The galvano-mirror of the second deflection means 211 isdriven by means of a saw-edged wave of a frequency of 60 Hz. Themodulated spots on the light receiving element are substantial circleshaving a diameter of about 40 μmø.

The deflected beam of light reflected by the reflector mirror 222 isconverged by the lens 209 to form a modulated spot on the lightreceiving element 101.

Modulation patterns are so defined that modulated spots are formed onthe light receiving element 101 in a manner as shown in FIG. 19B. Sincethe apparatus is adapted to two-dimensional scanning, the trajectory 104of the forwardly scanning modulated spot (scanning in direction A) andthe trajectory 104′ of the backwardly scanning modulated spot (scanningin direction B) are different from each other on the light receivingelement 101 as shown in FIG. 19B. Therefore, the forwardly scanningmodulated spot and the backwardly scanning modulated spot are separatedfrom each other and hence can be recognized with ease. In the instanceof FIG. 19B, it is possible to recognize that the upper modulated spot111 is the forwardly scanning one, whereas the lower modulated spot 111′is the backwardly scanning one.

The gap between the two trajectories and their respective inclinationschange depending on the positional arrangement of the light receivingelement 101 and the method employed for two-dimensional scanning(scanning trajectories do not necessarily agree with the horizontaldirection of the displayed image).

It is so arranged that an image is properly displayed on the projectionsurface by forward scanning and backward scanning when the positionaldisplacement between the forwardly scanning modulated spot and thebackwardly scanning modulated spot on the light receiving element 101(in terms of horizontal coordinate on the light receiving element 101)is found within a predetermined range 0±α as shown in FIG. 19B, where αis the tolerance of accuracy for forward scanning and backward scanningfor the image being displayed.

The above operation is controlled by the arrangement illustrated in FIG.5.

FIG. 20 is a flow chart of the operation of this example.

As the control starts, firstly an initial drive frequency and an initialmodulation pattern for detection are defined (S301). Firstly, thedeflection means 202 is driven on the basis of the above information togenerate a modulation pattern for detection.

The modulation pattern for detection is generated in such a way that itis turned on once for a forward scanning period and once for a backwardscanning period (S302). After the generation of the modulated spots, theelectric charge accumulated in the light receiving element istransferred by the modulated spots in such a way that the electriccharge in one of the plurality of light receiving regions is transferredat a time (S303).

Then, the positional relationship and the relative positionaldisplacement of the modulated spots are computed on the basis of thepositional information on the forwardly scanning modulated spot and thebackwardly scanning modulated spot (S304) by using the method describedabove by referring to the second embodiment for the computation.

If the computed relative positional displacement is found within thepredetermined tolerance range of ±α, the steps from S302 are repeated.If, on the other hand, it is not found within the predeterminedtolerance range, their positions in the horizontal direction of thelight receiving element 101 are compared with each other. Note that, therelative positional displacement is defined to be positive when theforwardly scanning modulated spot is found to the left of the backwardlyscanning modulated spot, whereas the relative positional displacement isdefined to be negative when the forwardly scanning modulated spot isfound to the right of the backwardly scanning modulated spot (S306).

If the positional displacement is positive, it is because the modulationtiming is too quick and hence the forwardly scanning modulated spot isfound to the left of the backwardly scanning modulated spot. Therefore,the drive frequency is raised so as to delay the phase of theresonance-type optical deflector proceed (S307).

If, on the other hand, the relative positional displacement is negative,it is because the modulation timing is too slow and hence the forwardlyscanning modulated spot is found to the right of the backwardly scanningmodulated spot. Therefore, the drive frequency is raised to makemodulation timing quicker (S308).

Thereafter, the processing operation returns to S302 to repeat the abovesteps.

In this way, the relative positional displacement of the forwardlyscanning modulated spot and the backwardly scanning modulated spot canbe held within a predetermined range when a resonance type opticaldeflector is used for the deflection means. Thus, there arises nodisplacement between image drawing of the forwardly scanning modulatedspot and that of the backwardly scanning modulated spot on projectionsurface so that an image is properly displayed. In other words, it ispossible to realize a projection-type image forming apparatus thatproduces fine images.

As two-dimensional scanning beams of light are detected by means of anarea sensor (light receiving element), the modulated spots can beseparated from each other and identified with ease. Additionally, thelight receiving element can be downsized and hence its cost can bereduced.

The light receiving element 101 is freed from restrictions of positionalarrangement because of the use of a reflector mirror 222 and a lens 209so that the entire apparatus can be downsized. Additionally, it ispossible to improve the detection accuracy.

As described above by way of example, the present invention can providean optical deflector that does not use a detection means for detectingthe time when a beam of light passes by a predetermined angle ofdeflection of a deflection means. Therefore, an optical deflectoraccording to the invention can very accurately control the operation ofthe deflection means in such a way that it is not affected by changes ofenvironmental temperature of the deflection means and the detectioncircuit.

1. An optical deflector having deflection means for deflecting modulatedlight from a light source so as to make deflected beams of light scan,said optical deflector comprising a control means for gauging a distancebetween a position of a deflected beam of light moving on a lightreceiving element in one direction and a position of another deflectedbeam of light moving on the light receiving element in the oppositedirection and controlling at least either said light source or saiddeflection means so as to make the distance agree with a predeterminedvalue.
 2. An optical deflector according to claim 1, further comprisingmeans for detecting a phase difference between a phase of the deflectedbeams of light and a phase predefined in said optical deflector from aposition of the deflected beam of light moving in the one direction anda position of the deflected beam of light moving in the oppositedirection and specifying means for specifying at least either a drivefrequency to be applied to said optical deflector or a modulation timingof forward and backward image drawing on the basis of the phasedifference.
 3. An optical deflector according to claim 1, wherein saidlight receiving element has a plurality of light receiving sections thatare arranged along the one moving direction of the deflected beam oflight.
 4. An optical deflector according to claim 1, wherein said lightreceiving element has a plurality of two-dimensionally arranged lightreceiving sections that are arranged along the one moving direction ofthe deflected beam of light and along a direction perpendicular to theone direction.
 5. An image forming apparatus comprising an opticaldeflector according to claim 1 and a light source as mentioned inclaim
 1. 6. An image forming apparatus according to claim 5, adapted todraw an electrostatic image on an electrophotography-type photosensitivebody by means of forwardly and backwardly moving deflected beams oflight.
 7. An image forming apparatus according to claim 5, adapted todraw an image on a projection surface by means of forwardly andbackwardly moving deflected beams of light.